1. Field of the Invention
The present invention is related to machinery having a relatively movable shaft, where a pressure-retaining seal establishes sealing contact with the relatively movable shaft to retain a pressurized first fluid and partition the first fluid from a second fluid having lower pressure. Examples of relatively movable shafts include shafts that have relative rotation with respect to the pressure-retaining seal, shafts that have relative reciprocation with respect to the pressure-retaining seal, and shafts that have both rotation and reciprocation relative to the pressure-retaining seal.
More specifically, the present invention is related to a sealing assembly that applies the pressures of the first and second fluids to controlled locations on a backup ring in order to provide relative immunity to pressure-induced diametric dimensional changes, and in order to provide opposed axially acting hydraulic forces that substantially balance one another, allowing the backup ring to align itself on the shaft and to follow lateral deflection and runout of the shaft. These benefits allow the backup ring to define a small initial extrusion gap for the pressure-retaining seal, minimizing pressure-induced extrusion damage to the pressure-retaining seal.
2. Description of the Prior Art
U.S. Pat. No. 5,195,754, entitled “Laterally Translating Seal Carrier for a Drilling Mud Motor Sealed Bearing Assembly,” and U.S. Pat. No. 6,227,547, entitled “High Pressure Rotary Shaft Sealing Mechanism” disclose axially force balanced seal carriers that follow the often unavoidable lateral motion of rotary shafts while maintaining a relatively small extrusion gap clearance between the seal carrier and the rotary shaft. The seal carriers of these patents require a step change in shaft diameter, and therefore cannot accommodate the significant shaft reciprocation found in some types of rotary equipment, such as the rotating control devices shown in U.S. Pat. Nos. 5,588,491 and 5,662,171. Another limitation with the seal carriers of U.S. Pat. Nos. 5,195,754 and 6,227,547 is that seals and seal carriers of different diameters are typically required for pressure staging.
The seal carriers of U.S. Pat. Nos. 5,195,754 and 6,227,547 are subject to the coning problem that is shown in FIG. 3 of Chapter D16 Rev. 0 (Apr. 23, 2012) of the Kalsi Seals Handbook, Rev. 6. Chapter D16 is titled “Axially force balanced, laterally translating arrangements,” and describes the problem as follows: “The portion of a laterally translating high pressure seal carrier that is located between the Kalsi-brand seal and the face seal experiences a radial pressure imbalance that causes an inward deflection of the seal carrier. This deflection has to be taken into account when designing the journal bearing fit, the extrusion gap fit, and the axial fit of the seal carrier with the surrounding support structure.” The problem with merely taking the deflection into account when designing the extrusion gap fit is that the extrusion gap closes as differential pressure builds, trapping extruded sealing material.
U.S. Pat. No. 6,007,105, entitled “Swivel Seal Assembly” provides pressure staging across a plurality of rotary seals engaging a relatively rotatable washpipe, and provides for simultaneous articulation of the washpipe and a surrounding seal housing to accommodate shaft run-out and misalignment. This simultaneous articulation is made possible by axial pressure balancing of both the washpipe and the seal housing. Because the seal housing expands due to internal pressure, the extrusion gap clearance changes as the sealed fluid pressure increases. Higher differential pressures could be withstood if the extrusion gap clearance could be held more constant.